A high speed photographic printer typically includes a light source under which a roll of developed film containing images on negatives is rapidly and continuously passed for reproducing the images on a roll of photosensitive paper, which is placed beneath the film. A lens assembly is placed between the paper and the roll of film for focusing the image and the amount of light which is directed onto the paper. The lens assembly includes two moveable groups of elements for controlling the magnification of the image and an iris diaphragm for controlling the amount of light.
The typical iris diaphragm includes a plurality of moveable blades for forming an adjustable aperture through which the light passes. The blades are typically controlled by a stepper motor which, via movement of the blades, induces the variation of the aperture. This configuration is discussed in detail in U.S. Pat. No. 4,444,489.
Although the above described system and method for varying the aperture are satisfactory, they are not without drawbacks. It takes approximately two or three seconds to adjust the diameter of the aperture, and for high speed photographic printers, such a response time is insufficient to allow the aperture to change between images on the same roll of film because of the rapid speed at which the film is moving under the light source. High speed photographic printers typically require a quick response time, approximately 50 milli-seconds between images. Therefore, the entire roll of film is printed at one aperture setting, and if changes are needed for any of these printed images, the aperture is then adjusted and the copying process repeated. This consumes time, which obviously adds additional cost to the printing process.
In addition, the stepper motor and its associated blades should be replaced after approximately two million actuations because experience has shown this to be its expected lifetime. For high speed photographic printers, two million actuations are consumed in a short period of time. For this reason, the durability of the stepper motor arrangement is inadequate for high speed photographic printers.
Finally, the stepper motor arrangement consumes a great amount of space. This is obviously undesirable because it increases the size of the photographic printer.
Consequently, a need exists for an iris diaphragm having a fast response time for permitting aperture variations between images, and having improved durability for reducing maintenance and replacement.